Pheochromocytomas are tumors of chromaffin cells that synthesize and secrete catecholamines. This project tests the efficacy of radiotoxic treatment of malignant pheochromocytoma using 131I-metaiodobenzylguanidine (131I-MIBG), and in particular tests whether pre-treatment "sensitization" increases the efficacy of 131I-MIBG treatment in reducing the size and number of tumors and the tumor burden. The central hypothesis is that in patients with malignant pheochromocytoma, treatment with 131I-MIBG destroys tumor cells and thereby decreases the size and number of tumors and the tumor burden; and that the "sensitization" procedures augment the effectiveness of 131I-MIBG. This new protocol still needs some clarifications from the FDA, therefore, no patients have been enrolled into the study so far.